MDL-81725 core_courseformat: Prevent drag and drop handling subsections

* Subsection are "modules" and should not be handled as "sections" in the
drag and drop process. Preventing delegated section from being dragged in
the dndsectionitems module fixes the drag and drop issues with subsections
in course index.

course/format/amd/src/local/courseeditor/dndsectionitem.js

@@ -108,6 +108,9 @@ export default class extends BaseComponent {
      * @returns {Object} exported course module drop data
      */
     _getDraggableData() {
+        if (this.section.component !== null) {
+            return null;
+        }
         const exporter = this.reactive.getExporter();
         return exporter.sectionDraggableData(this.reactive.state, this.id);
     }